The product of the bacteriophage T4 pseT gene is both a 5' polynucleotide kinase and a 3' phosphatase. Because T4 pseT mutants show reduced rates of T4 late gene expression under some conditions we think this enzyme is involved in T4 late transcription. As part of an attempt to further understand the biological role of the T4 induced polynucleotide 5' kinase, 3' phosphatase we isolated mutants of E. coli K12 which will support the development of wild type but not pseT deletion mutants of T4. This led to the discovery of one, and possibly two, E. coli genes whose products participate in T4 late gene expression. We named these the lit genes of E. coli. Experiments are proposed to test the hypothesis that the product of one of the lit genes is an enzyme analogous to the T4 induced polynucleotide 5' kinase 3' phoshatase. Also proposed are experiments to study the functions o the host and T4 gene products we think are involved in the same reaction as the T4 pseT gene product. There are T4 mutants which multiply on host lit mutants. We have named them Go1 mutants for Grow on lit. We think gol mutations alter a site at which T4 DNA attaches to some other cellular structure, perhaps membrane. This attachment is required for the expression of the T4 late genome and for proper DNA packaging into virus particles. Experiments are proposed to further characterize gol mutations and isolate and sequence the gol site in wild type T4 and some of our mutants.